The present invention relates to a flow metering apparatus utilizing Karman's vortex, and more particularly to an improved apparatus which measures the velocity or quantity of a flow by detecting an alternating force caused in a vortex generator due to Karman's vortex and producing a vortex signal therefrom.
In general, when an object such as round bar is inserted into a fluid, stable and regular vortexes are generated on the left and right alternately to the flow in the downstream of the object. Such a vortex results from exfoliation of a boundary layer on the object surface and is called Karman's vortex.
It is well known heretofore that the number of vortexes (vortex generation frequency) generated behind the object during a unit time is proportional to the flow velocity of the fluid. Therefore, the flow velocity or quantity of the fluid can be obtained if the number of the vortexes generated during the unit time is found.
The flow metering apparatus according to the present invention detects an alternating force caused in a vortex generator due to such Karman's vortex and produces a vortex signal therefrom to measure the flow velocity or quantity.
The prior art for vortex signal detection known heretofore include a heat sensing system and a strain detecting system disclosed in the U.S. Pat. No. 3,972,232, in which various problems are left unsolved individually. In the heat sensing system, there occurs a sudden reduction of sensitivity if any slight dust or the like adheres to a thermistor or a heat sensing element such as platinum wire. And in the strain detecting system, for the purpose of causing a strain with high sensitivity by a small alternating force resulting from Karman's vortex, it is necessary to compose the vortex generator of a flexible material or to form a flexible region in the middle portion where the vortex generator is supported in a duct, so as to bond a strain detecting element thereto. And yet the strain on the object surface is required to be great for ensuring a sufficiently high detection sensitivity while the rigidity of the sensor should be small. In the U.S. Pat. No. 3,972, 232, the vortex generator is supported firmly in the duct, while the sensor is supported flexibly in a chamber formed in the vortex generator and is isolated from outside by means of a diaphragm, with the chamber being filled with oil. Accordingly, a solid construction is not attainable due to flexibility of the sensor and the diaphragm. Moreover, because of oil, there is restriction that prohibits operation in a high temperature range even with selective use of silicone oil or the like which is relatively resistant against high temperature.
The present invention is so constituted that the sensor is substantially integrated with the vortex generator for directly detecting the alternating force produced in the vortex generator. This eliminates the flexible parts and remarkably simplifies the construction with resultant solidity and durability while offering high sensitivity and stability with a merit of widening the working temperature range.